Local circuit association neurons in the superficial dorsal horn subserve important functions in the gating and transduction of somatosensory information from the periphery. As such, these neurons are important components of pain pathways in the spinal cord. We have recently shown dorsal association interneurons are derived from an embryonic population of cells that express Lbxl (Gross et al., 2002). However, very little is known about how these spinal neurons differentiate to form the many specialized functional cell types that are found in the adult spinal cord. In this grant, we will examine how Lbx 1 neurons are specified, focusing on the role of the Gshl, Gsh2 and Gbx2 homeobox genes in Lbxl neuron progenitors. We will also determine how late-born Lbx I neurons differentiate as either inhibitory or excitatory neurons, and ask whether Notch signaling regulates the developmental switch that selects between excitatory and inhibitory cell fates. The observation that Pax2 is required for dorsal GABAergic and glycinergic development makes these dorsal horn interneurons an excellent model system for probing the molecular mechanisms that regulate the acquisition of an inhibitory neurotransmitter phenotype. We will examine the role that the Pax2, Lhxl/5 and the Ets2 transcription factors play in the development of dorsal inhibitory cell types. In particular, we will ask whether these factors, or other factors that are downstream of Pax2, regulate the expression of genes that are required for the inhibitory transmission machinery. Finally, we will generate conditional Pax2 knockout mice and use these mice to test whether Pax2 has a 'late' function in dorsal inhibitory neurons. We will also use the Pax2 conditional KO mice to begin assessing the function of GABAergic neurons in the dorsal horn with respect to the transduction of noxious and non-noxious stimuli, as well as the development of pain neuropathies such as hyperalgesia. These studies will provide important insights into the development and organization of neural circuits in the dorsal horn that sense and relay pain. [unreadable] [unreadable] [unreadable]